This invention relates to apparatus for extracting mechanical power from wind. More particularly, it relates to apparatus of the nature of a flying windmill and means for transmitting the mechanical power from that flying windmill to the ground.
The best known type of device for generating mechanical power from the wind at a ground location is the conventional, tower-mounted windmill or wind turbine. While the tower-mounted wind turbines have proved satisfactory in many applications, they are generally limited by their relatively close proximity to the ground, thus resulting in their receiving winds of substantially lower speed and energy than are available higher in the air. In the prior art it has been recognized that improved performance can be obtained by aerodynamic support of the power producing structure, as disclosed in U.S. Pat. No. 3,924,827, U.S. Pat. No. 3,987,987, U.S. Pat. No. 4,076,190 and U.S. Pat. No. 4,084,102. These and other prior art have disclosed various types of unmanned kites controlled by the ground and containing some type of turbine for extraction of power from movement of the air over the kite.
It has been known tht a kite may be aerodynamically supported either facing into the wind or tacking across the wind. When tacking across the wind, the kite velocity through the air may approximate the product of the wind speed and the lift-to-drag ratio of the vehicle or kite. If the kite is an aerodynamic lifting vehicle, its lift increases as a square of its velocity. As a result, the vehicle can pull larger loads and move the loads faster when travelling across the wind. By this increased speed, a turbine mounted to the kite vehicle may be caused to turn far faster than if the vehicle were simply facing into the wind, thus providing for the extraction of substantially greater power from the wind.
Disadvantages have arisen in the prior art in which a generator is carried on board the kite vehicle and driven by the wind turbine, since the weight of such a generator on board the vehicle may severely compromise its aerodynamic performance. In other prior art structures power has been extracted from the movement of the tether about a ground mounted base point, suffering the limitation that the power production is synchronously tied to the motion of the kite about the base point. The use of a rotating tether supported by a stationary vehicle suffers the disadvantage that extraction of the power from the wind by the turbine results in substantial drag, thus pulling the tether down to a low angle to the ground and reducng both its efficiency and its exposure to the higher velocity winds at greater altitude.